1. Field of the Invention
The present invention relates to an ignition timing control apparatus for an internal combustion engine used for, for instance, an outboard machine.
2. Discussion of Background
In a conventional ignition timing control apparatus, a crank angle was detected on the basis of a signal from a crank angle sensor which produces every 1.degree., for instance, a signal, and the ignition of engine was performed at the optimum angle on the basis of the detected crank angle.
In the conventional ignition timing control apparatus, however, when the crank angle sensor became abnormal because of disconnection of wire, a short circuit or the like, information of the crank angle was not provided and ignition control became impossible, resulting in an engine stop. If such problem occurs in an outboard machine with the ignition timing control apparatus in a ship on the sea, there is a danger that the ship can not return to a harbor.